The present invention relates to a circuit for synchronizing clock signals and can be applied where recording and reproducing sections are synchronized with each other when digital data in one recording medium are copied on to another recording medium in a device which is intended to record and reproduce digital data.
In conventional analog tape recorders, for example, changes in the amplitude of signals, such as audio signals, which represent analog data are reflected in changes in the strength of magnetic fields on a magnetic tape. The performance of the head, tape and other parts of such recorders influence the dynamic range, distortion factor, and other characteristics of the audio signals. In addition the performance of a tape transporting system influences the frequency characteristics of the audio signals like wow, fluttering phenomenon, and introduced noise, and the audio signals are modulated by, for example the fluctuation of tape speed. Therefore, analog recorded audio information is degraded in tone quality every time it is copied.
There has been developed a system which records audio signals as digital data and reproduces the recorded digital data. This system samples audio signals at a predetermined sampling frequency, quantizes the amplitude of sampled audio signals, and records the audio signals as digital data on the tape. If the logical level of information recorded on the tape can be precisely determined to be either "0" or "1" when records stored on the tape are reproduced, the original analog waveforms will be restored completely. In actual practice the recording medium on which digital data are recorded is sommetimes deformed after it is used many times. In addition, errors are sometimes caused on recorded contents. Accordingly, recorded contents must be sometimes copied on a new recording medium. The reproducing device which copies digital data has usually error correcting function, thus allowing error-corrected digital data to be copied on the new recording medium. Therefore, unlike the copying of analog data, even an audio signal of low frequency can be restored without being influenced by the speed fluctuation wow, flutter, etc., of reproducing system.
When copying digital data onto another recording medium, digital data are transferred between reproducing section and recording section. It is therefore necessary that the reproducing section is completely synchronized with the recording section. Namely, a clock signal employed in the reproducing section and a clock signal employed in the recording section must be phase sychronized.